General System Description

DESCRIPTION
The diagnostic Tables and functional checks are designed to locate a faulty circuit or component through a process of logical decisions. The Tables are prepared with the assumption that the vehicle functioned correctly at the time of assembly and that there are not multiple faults present.

There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures contained in System Diagnosis. The language of communicating the source of the malfunction is a system of diagnostic trouble codes. When a malfunction is detected by the control module, a diagnostic trouble code will set and the Malfunction Indicator Lamp (MIL) Malfunction Indicator Lamp will illuminate on some applications.

MALFUNCTION INDICATOR LAMP (MIL)
The MIL (Malfunction Indicator Lamp) is on the instrument panel. The MIL has the following functions:

^ The MIL informs the driver that a fault that affects the vehicle's emission levels has occurred. The owner should take the vehicle for service as soon as possible.
^ As a bulb. and system check, the MIL (Malfunction Indicator Lamp) comes on with the key on and the engine not running. When the engine is started, the MIL turns off if no DTCs are set.

When the MIL remains on while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, perform an On-Board Diagnostic (OBD) System Check. The procedures for these checks are given in engine controls. These checks expose faults which the technician may not detect if other diagnostics are performed first. Testing and Inspection

INTERMITTENT MALFUNCTION INDICATOR LAMP
In the case of an intermittent fault, the MIL (Malfunction Indicator Lamp may illuminate and then after 3 trips turn off. However, the corresponding diagnostic trouble code will store in the memory. When unexpected diagnostic trouble codes appear, check for an intermittent malfunction.

DATA LINK CONNECTOR
The provision for communicating with the control module is a Data Link Connector (DLC). The DLC is usually located under the instrument panel. The DLC is used in order to connect to a scan tool. Some common uses of the scan tool are listed below:

^ Identifying stored Diagnostic Trouble Codes (DTCs)
^ Clearing the DTCs
^ Performing output control tests
^ Reading serial data

CONTROL MODULE LEARNING ABILITY
The control module has a "learning" ability which allows the control module to make corrections for minor variations in the fuel system in order to improve driveability. Whenever the battery cable is disconnected, the "learning" process resets. The driver may note a change in the vehicle's performance. In order to allow the PCM to "re-learn" to drive the vehicle at part throttle with moderate acceleration. The vehicle may also operate at idle conditions until the normal performance returns.

REPROGRAMING (FLASHING) THE CONTROL MODULE
Some vehicles allow the reprogramming of the control module without removal from the vehicle. This provides a flexible and a cost-effective method of making changes in software and calibrations. Refer to the latest Techline information on re-programming or flashing procedures.